1. A. Einstein, Collected Scientific Works, Vol. 1 [Russian translations], Nauka, Moscow (1965).
2. E. Schrödinger, Phys. Z., No. 19, 4 (1918).
3. D. Hilbert, Göttingen Nachrichten,4, 21 (1917).
4. Ya. B. Zel'dovich and L. P. Grishchuk, Usp. Fiz. Nauk.,149, 695 (1986).
5. L. D. Landau and E. M. Lifshitz, The Classical Theory of Fields, 4th ed., Pergamon Press, Oxford (1975).